When exposed to radiation such as X-rays, an energy-storing phosphor (e.g., stimulable phosphor, which gives stimulated emission off) absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when it is exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storing phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storing phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the panel; sequentially scanning the storage panel with a stimulating light such as a laser beam to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) has a basic structure comprising a support and an energy-storing phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a protective layer is normally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical damage.
The phosphor layer generally comprises a binder and an energy-storing phosphor dispersed therein. However, the phosphor layer may comprise agglomerate of an energy-storing phosphor without binder, and in fact that phosphor layer is already known. The phosphor layer containing no binder can be formed by a vapor phase deposition method or by a firing method. Further, the phosphor layer may comprise energy-storing phosphor agglomerate impregnated with a polymer material, which is already known, too.
Japanese Patent Provisional Publication 2001-255610 discloses a variation of the radiation image recording and reproducing method. While an energy-storing phosphor of the storage panel used in the ordinary method plays both roles of radiation-absorbing function and energy-storing function, those two functions are separated in the disclosed method. In the method, a radiation image storage panel comprising an energy-storing phosphor (which stores radiation energy) is used in combination with a phosphor screen comprising another phosphor which absorbs radiation and emits ultraviolet or visible light. The disclosed method comprises the steps of causing the radiation-absorbing phosphor of the screen (and of the panel) to absorb and convert radiation having passed through an object or having radiated from an object into ultraviolet or visible light; causing the energy-storing phosphor of the panel to store the energy of the converted light as radiation image information; sequentially exciting the energy-storing phosphor with a stimulating light to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric signals giving a visible radiation image.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. However, it is still desired that the radiation image storage panel used in the method have a sensitivity as high as possible and, at the same time, give a reproduced radiation image of high quality (in regard to sharpness and graininess).
For improving the sensitivity of radiation image storage panel, a light-reflecting layer is often provided between the support and the phosphor layer. Japanese Patent Provisional Publication 2001-124898 discloses a radiation image storage panel in which a light-reflecting layer is provided on one side of the phosphor layer containing a stimulable phosphor. The light-reflecting layer contains light-reflecting material, and shows a scattering length of 5 μm or less at the wavelength where a stimulating light excites the stimulable phosphor. By providing a light-reflecting layer showing such a short scattering length, the storage panel is improved not only in the sensitivity but also in the sharpness. That is because the stimulating light is reflected with enough reflectivity to prevent from diffusing, and accordingly the storage panel gives a reproduced radiation image of high quality with high sensitivity.
In order to prepare a light-reflecting layer giving the above-described short scattering length, it is necessary to pack densely the light-reflecting material in the form of fine particles (i.e., it is necessary to lower the ratio of binder/reflecting material in the light-reflecting layer) in the reflecting layer. However, if a coating solution for the reflecting layer contains a relatively large amount of the reflecting material, a coating solution suffers from various troubles. For example, the particles of reflecting material are prone to be insufficiently dispersed; the resultant reflecting layer is apt to have uneven thickness; and defects (e.g., lines or spots due to unevenly spread coating solution) are often formed in the coated layer. The insufficient dispersion lowers reflectivity of the reflecting layer, uneven layer thickness disadvantageously makes image-signals with such uneven S/N ratios that the resultant storage panel has uneven sensitivity, and the troubles occurring in the coating procedure increase the production cost.